The present invention relates to an alarm device or system responsive to trespassing used for prevention of crime, and more particularly to a device for effecting detection of attempted entering of premises by an unauthorized person and the like with infrared rays (which are also called "heat rays"), thereby to produce alarm signals responsive thereto.
Generally, there have been proposed various kinds of burglary preventing devices. Particularly, when, an unauthorized person such as a man is a target to be detected, a method of detecting infrared rays radiated from the human body is effective for this purpose. In accordance with this method, alarm signals are produced on the basis of the amount of infrared rays sensed and the state of the change thereof, when a person passes within the range of vision monitored by the infrared ray sensor.
However, with conventional alarm devices of the type stated above, the following drawback is pointed out. Namely, it happens that alarm signals are erroneously produced, even if there is no unauthorized person or trespasser attempting to enter a room, due to the influence of a draft admitted thereinto, a change in temperature within a room occurring due to an air-conditioner, such as, a cooler or heater, or a fan is switched off, or a change in room temperature which is caused depending on the temperature in the morning or in the evening.
As stated above, if alarm signals are erroneously produced many times, the reliability for alarm devices is lowered or lost.